Exhaust gas from internal combustion engines of vehicles and the like contain hydrocarbons, carbon monoxide, and nitrogen oxide, which are harmful to human body and environment. As a catalyst for purifying vehicle exhaust gas, so-called a three way catalyst is used, which oxidizes carbon monoxide and hydrocarbons into carbon dioxide and water, and reduces nitrogen oxide into nitrogen and water. A three way catalyst is composed, for example, of precious metals Pt, Pd, and Rh as a main catalyst, and an oxide or a composite oxide containing cerium oxide as a co-catalyst, both carried on a catalyst support of alumina, cordierite, or the like. A co-catalyst absorbs oxygen due to change of valency of Ce contained therein from three to four under an oxidizing atmosphere, and desorbs oxygen due to change of the cerium valency from four to three under a reducing atmosphere, which is so-called oxygen absorbing and desorbing capability. This oxygen absorbing and desorbing capability mitigates abrupt change in an exhaust gas atmosphere caused by acceleration and deceleration of an engine, so as to allow the main catalyst to purify exhaust gas at high efficiency. As a co-catalyst, composite oxides containing Ce and Zr are widely used. However, currently used composite oxides with Ce and Zr do not have sufficient oxygen absorbing and desorbing capability. In particular, the amount of oxygen absorption and desorption of these composite oxides is small at lower temperatures of 400° C. or lower, so that no mitigation of change in an exhaust gas atmosphere is exhibited when the engine temperature is low, e.g., at the engine start, which makes the exhaust gas purifying effect of the main catalyst low.
On the other hand, development of polymer electrolyte fuel cells (PEFC) has advanced, but a high cost of platinum catalyst, which is used both in anodes and cathodes, impedes practical application and popularization of PEFC. The oxygen reduction reaction at the cathode, 1/2O2+2H++2e−→H2O, particularly requires a large amount of platinum catalyst. Thus catalyst materials have been actively developed which can substitute or reduce the amount of platinum catalyst to be used.
As a composite oxide usable as a catalyst material, for example, Patent Publication 1 discloses a composite oxide having oxygen absorbing and desorbing capability, which contains cerium oxide, zirconium oxide, and hafnium oxide, the composite oxide including φ′ phase as a crystal phase and having oxygen absorbing and desorbing capability of at least 100 μmol/g at 400 to 700° C.
Patent Publication 2 discloses a zirconium-cerium composite oxide containing zirconium and cerium at a weight ratio in the range of 51 to 95:49 to 5 in terms of oxides, the composite oxide having a specific surface area of not smaller than 50 m2/g after calcination at 500 to 1000° C., and capable of maintaining a specific surface area of not smaller than 20 m2/g even after heating at 1100° C. for 6 hours.
Patent Publication 3 discloses use of a composite oxide as a co-catalyst, the composite oxide being composed of praseodymium oxide and zirconium oxide, and having a specific surface area of as large as 10 m2/g or more. The disclosed composite oxide has high oxygen absorbing and desorbing capability even at low temperatures of 200° C. to 350° C.
Patent Publication 4 discloses an exhaust gas purifying catalyst having a honeycomb support and a catalyst layer formed thereon which contains a Zr—Pr composite oxide carrying precious metals. The disclosed exhaust gas purifying catalyst is characterized by providing a low light-off temperature of hydrocarbons.
Patent Publication 5 discloses use of a composite oxide composed of Ce, and Pr or Tb, and Zr as a co-catalyst. The disclosed composite oxide exhibits good oxidation-reduction capability even in exhaust gas generated by combustion of a lean (fuel is rarefied) air-fuel mixture.    Patent Publication 1: JP-8-109020-A    Patent Publication 2: JP-10-194742-A    Patent Publication 3: JP-2001-113168-A    Patent Publication 4: JP-2006-68728-A    Patent Publication 5: JP-2000-72447-A
The composite oxides disclosed in Patent Publications 1 to 5, however, do not yet have sufficient oxygen absorbing and desorbing capabilities. In particular, the amount of oxygen absorption and desorption at low temperatures of 400° C. or lower is still small.